Light fibers and methods for producing the same

ABSTRACT

A light fiber comprising (a) a tubular clad having a predetermined length and (b) a solid core formed by reacting a filler material with which the clad is filled, the clad and the core being closely contacted with each other by shrinkage of the clad, characterized in that: the clad is expandable under pressure, shrinkage of the clad is initiated before the completion of the reaction of the filler material and is carried out in conformity with volume reduction of the core, which accompanies the reaction of the filler material, and the number of air gap between the clad and the core, which is measured per length of 10 m, is 3 or less.

TECHNICAL FIELD

[0001] The present invention relates to a light fiber and a method ofproducing the same and, more particularly, to a light fiber comprising(a) a tubular clad having a predetermined length, which is expandableunder pressure and (b) a solid core formed by reacting a filler materialwith which the clad is filled, the clad and the core being closelycontacted with each other by shrinkage of the clad, and a method ofproducing the same, particularly a method of producing a light fiber,capable of effectively preventing generation of air gap between the cladand the core.

BACKGROUND OF THE INVENTION

[0002] There has been known a method of filling a tubular clad with apolymerizable monomer capable of forming a solid core after thecompletion of the reaction (particularly polymerization reaction), andpolymerizing the monomer to produce a light fiber. In case the lightfiber is produced by such a method, it is usually important tosubstantially prevent air gap (also referred to as an internal defect)such as cellular defect, peel or the like from generating between thecore and the clad (interface). Since such air gap lowers the lighttransmission efficiency of the light fiber, it becomes impossible totransfer light incident upon the core from one end to the other end witha sufficient dose of light in the comparatively long size light fiber(having a length of 20 m or more).

[0003] As the method of producing the light fiber, capable of inhibitinggeneration of such air gap as possible, for example, a method of fillingwith a monomer under pressure (disclosed in Japanese Unexamined PatentPublication (KOKAI) No. 57-45502), a method of successively polymerizinga monomer from one end to the other end (disclosed in JapaneseUnexamined Patent Publication (KOKAI) No. 7-168029) and a method ofpolymerizing a monomer while applying a pressure in an axial directionof a clad (disclosed in Japanese Unexamined Patent Publication (KOKAI)No. 7-168028). There is also known a method of covering a previouslythermally-expanded clad with a jacket, polymerizing a monomer, removingthe jacket, and shrinking the clad by applying heat (disclosed inJapanese Unexamined Patent Publication (KOKAI) No. 2-306205). In theabove mentioned successive polymerization method, there is usuallyemployed a so-called batch-wise manufacturing system of filling atubular clad, whose one end is sealed, with a monomer under pressure,fixing the clad filled with the monomer in a heating bath, andpolymerizing the monomer by heating. At this time, the monomer ispolymerized successively from one end to the other end by raising aheating temperature (i.e. temperature of the heating medium in theheating bath).

[0004] The formers of a series of methods described above, i.e. threemethods utilizing filling with monomer under pressure, successivepolymerization and axial pressurization of clad, have an effect ofpreventing generation of air gap at the interface between the core whosevolume was reduced by polymerization, and the clad, by filling with themonomer in the amount corresponding to the volume reduction, occurred onpolymerization, from the unpolymerized side.

[0005] On the other hand, the latter method has an effect of thermallyexpanding a clad, reversibly, and shrinking an internal radius of theclad corresponding to an external diameter of the core whose volume wasreduced after polymerization, thereby enhancing the adhesion between thecore and the clad.

[0006] According to the former three methods among conventional methodsdescribed above, in case the region of interface between theunpolymerized portion where the monomer retains mobility and thepolymerized portion where the monomer was solidified to lose themobility is comparatively narrow (volume reduction of the monomer occursonly within a range where an influence of the pressure from theunpolymerized portion is exerted), it is possible to fill with themonomer in the amount corresponding to the volume reduction, occurred onpolymerization, from the unpolymerized side. However, these methods arenot effective for the case where the light fiber is continuouslyproduced, for example, by the steps of:

[0007] (I) transferring a clad filled with a monomer to a heating regionsuch as temperature-controlled bath using a feeding means,

[0008] (II) initiating, carrying out and completing the reaction of themonomer in the heating region, and

[0009] (III) passing the resulting light fiber after the completion ofthe polymerization reaction through the heating region and winding thelight fiber, using a winding means disposed outside the heating region.That is, in case the productivity is enhanced by increasing thepolymerization reaction rate, it becomes impossible to obtain asufficient monomer filling effect because the region of the interfacebetween the unpolymerized portion and the polymerized portion (portionwhere the polymerization has been completed) forms a boundary portionhaving a certain width and the volume reduction of the monomer occurs atthe portion where an influence of the pressure from the unpolymerizedportion is not exerted. Accordingly, it is difficult to stably produce alight fiber which is substantially free from air gap (the number of airgap between the clad and the core, which is measured per length of 10 m,is 3 or less) in case of continuous production.

[0010] In case a batch-wise production of the light fiber is carried outby using such a method, shrinkage of the external diameter of the coreis compensated by shrinkage of the core in the lengthwise direction,thereby making it difficult to produce a comparatively long size lightfiber (having a length of 20 m or more).

[0011] On the other hand, in the method of covering a previouslythermally-expanded clad with a jacket, completing the polymerization ofa monomer, and removing air gap between the core and the clad, whichaccompanies the polymerization, by shrinkage of the clad with heating,the production process is complicated when continuous production isconducted. Also, in case of removing air gap, which has already beenformed, in the continuous processing of a comparatively long size lightfiber, there is a considerable fear that the gas from the air gapportion can not escape from the other end portion of the light fiber andis trapped in the halfway of the light fiber, resulting in formation ofdefects.

[0012] Thus, an object of the present invention is to provide a longsize light fiber (having a length of 20 m or more), which issubstantially free from air gap between the core and the clad and issuperior in light transmission performance.

[0013] Another object of the present invention is to provide a method ofproducing a light fiber, capable of producing such a light fiber in astable and easy manner.

SUMMARY OF THE INVENTION

[0014] According to the present invention, the problems described abovecan be solved by a light fiber comprising (a) a tubular clad having apredetermined length and (b) a solid core formed by reacting a fillermaterial with which the clad is filled, the clad and the core beingclosely contacted with each other by shrinkage of the clad,characterized in that:

[0015] the clad is expandable under pressure,

[0016] shrinkage of the clad is initiated before the completion of thereaction of the filler material and is carried out in conformity withvolume reduction of the core, which accompanies the reaction of thefiller material, and

[0017] the number of air gap between the clad and the core, which ismeasured per length of 10 m, is 3 or less.

[0018] According to the present invention, there is also provided amethod of producing a light fiber comprising (a) a tubular clad having apredetermined length and (b) a solid core formed by reacting a fillermaterial filled in the clad, the clad and the core being closelycontacted with each other by shrinkage of the clad, said methodcomprising the steps of:

[0019] forming the clad which is expandable under pressure,

[0020] filling the clad with the filler material and reacting the fillermaterial in the clad while applying a pressure,

[0021] initiating a shrinking operation of the clad before thecompletion of the reaction of the filler material, and

[0022] shrinking the clad with volume reduction of the core, whichaccompanies the reaction of the filler material, thereby inhibitinggeneration of air gap between the clad and the core.

BRIEF DESCRIPTION OF THE DRAWING

[0023]FIG. 1 is a schematic view showing one preferred embodiment of aproduction line of the light fiber according to the present invention.

DETAILED DESCRIPTION

[0024] The light fiber of the present invention is characterized inthat:

[0025] the clad is expandable under pressure,

[0026] shrinkage of the clad is initiated before the completion of thereaction of the filler material and is carried out with volume reductionof the core, which accompanies the reaction of the filler material, and

[0027] the number of air gap between the clad and the core, which ismeasured per length of 10 m, is 3 or less. As is easily understood fromthe following description, these characteristics are particularlyderived from the method of producing the light fiber according to thepresent invention.

[0028] According to the present invention, in the step of forming asolid core by polymerizing a filler material such as monomer in atubular clad while applying a pressure, a shrinking operation of theclad is initiated before the completion of the reaction of the core(monomer), and then the clad is shrunk with volume reduction of thecore, which accompanies the reaction of the filler material, therebyinhibiting generation of air gap between the clad and the core. That is,according to the method of the present invention, the light fiber isproduced in the state where the filler material such as monomer isalways closely contacted with the core formed by polymerizing the fillermaterial and the clad. Therefore, it is possible to produce a lightfiber, which is substantially free from air gap between the clad and thecore, in a stable and easy manner even in case of continuous production.

[0029] As used herein, “the state which is substantially free from airgap” means that the number of air gap between the clad and the core,which is measured per length of 10 m, is 3 or less. The number of airgap is preferably 2 or less, and usually 1 or less. The air gap is adefect which can be visually recognized in case of incidence of lightupon the fiber (core), and the maximum size is usually 5 mm or more.

[0030] As used herein, “the maximum size” refers to a diagonal line incase of a defect in a generally polygonal shape, and a major axis incase of a defect in a generally elliptical shape. The method ofmeasuring the defect is preferably a method of counting those which canbe visually recognized as a dark spot on the fiber side in case ofincidence of light from one end of the light fiber.

[0031] The clad used in the light fiber of the present invention is atubular one which has a predetermined length and is expandable underpressure. Accordingly, it can be easily shrunk with volume reduction ofthe core, which accompanies the reaction of the filler material. Such aclad is preferably made of a material which is expandable by causingplastic or elastic deformation at a predetermined temperature (e.g.reaction initiating temperature of filler material) under pressure andis thermally expandable by causing plastic or elastic deformation at aheating temperature at which the filler material is reacted.

[0032] Selection of the clad as described above is preferably conductedby appropriately deciding values of main physical properties, i.e.thickness of clad, internal radius of clad and elastic modulus of cladmaterial, according to the pressure for pressurizing the filler materialin the clad and temperature (temperature of polymerization bath) atwhich the filler material is reacted.

[0033] In other words, since expansion of the clad and its shrinkagewith volume reduction of the core, which accompanies the reaction, canbe conducted in a secure and stable manner (uniformly) by controllingthe conditions as described above, there can be produced a long sizelight fiber, which is substantially free from air gap between the coreand the clad and is superior in light transmission performance, in astable and easy manner.

[0034] According to the present invention, continuous production of thelong size light fiber can be efficiently conducted. One method suitedfor continuous production is a method described below. That is, it is amethod of continuously producing a light fiber, comprising the steps of:

[0035] (A) feeding a clad from a feeding means, on which the clad iswound, using winding means, thereby transferring a filler material(polymerizable monomer), with which the clad is filled, to a heatingregion, said clad extending continuously in a lengthwise direction sothat it can be fed and wound,

[0036] (B) initiating, carrying out and completing the reaction of thefiller material in the heating region,

[0037] (C) initiating a shrinkage operation of the clad in the heatingregion, and

[0038] (D) winding the resulting light fiber after the shrinkingoperation of the clad has substantially been completed, using a windingmeans disposed outside the heating region.

[0039] This method and a conventional batch-wise method aresubstantially the same in that a polymerization bath having apredetermined length, which includes as the heating region a heatingmedium such as water, is used. However, according to the aboveproduction method, the polymerization reaction operation of the monomerand the shrinking operation of the clad can be effectively conducted,simultaneously, by continuously passing the clad filled with the monomerthrough the heating region along the lengthwise direction of thepolymerization bath, thereby making it possible to continuously producta long size light fiber, which is substantially free from air gap and issuperior in light transmission performance, in a stable and easy manner.According to the this method, since the light fiber can be produced byusing a clad having a length of 100 m or more, the productivity can beeasily enhanced. From such a point of view, it is preferred that theclad filled with the filler material is fed to the heating region whilethe clad is continuously filled with the filler material under pressure,and then continuously reacted.

[0040] In the light fiber produced in the method of the presentinvention, it is very easy to reduce the number of air gap between theclad and the core, which is measured per length of 10 m, to 3 or less byshrinkage of the clad with volume reduction of the core, whichaccompanies the reaction of the filler material.

[0041] In case light is transmitted from one end to the other end, thelight fiber can have sufficient light transmission performance forpractical use if the number of air gap is 3 (per 10 m) or less. In casethe light fiber is used for decoration utilizing light to be leaked fromthe fiber side, the number of air gap is 1 (per 10 m) or less. The lightfiber according to the present invention can have sufficient lighttransmission performance for practical use in case of a product having acontinuous core of 20 to 100 m.

[0042] Next, the light fiber according to the present invention as wellas preferred method and conditions for production thereof will bedescribed in detail.

[0043] [Method of Producing Light Fiber]

[0044] Preferred method of producing a light fiber according to thepresent invention will be described in a stepwise manner.

[0045] First, a tubular clad (clad tube) is prepared. Usually, the cladis formed by an extrusion molding method so as to have a predeterminedsize such as thickness, internal radius, length or the like. Thematerial and size of the clad will be described hereinafter.

[0046] The clad 1 formed as described above is usually set to a feedingapparatus 11 as a feeding means, as shown in FIG. 1. The feedingapparatus 11 shown in the drawing is equipped with a rotatably disposedcore tube (not shown) and is set after winding around the core tube. Theclad 1 set to the core tube of the feeding apparatus is usually wound bydriving a winding apparatus 12 bas a winding means, as shown in thedrawing. The winding apparatus 12 shown in the drawing is equipped witha rotatably disposed core tube (not shown) and a driving means (e.g.motor, not shown) for rotating the core tube.

[0047] Using the feeding apparatus 11 and the winding apparatus 12, asshown in the drawing, in combination, the clad 1 extending continuouslyin the lengthwise direction is fed to a heating bath 13 (water bath forpolymerization in the embodiment shown in the drawing) disposed betweenthe feeding apparatus 11 and the winding apparatus 12 at a predeterminedfeed rate, and then passed through the bath.

[0048] The heating bath 13 is usually equipped with a container capableof being filled with a heating medium such as water, oil, air or thelike, a heating means (e.g. heater) for heating the heating medium, anda temperature controlling apparatus (e.g. thermostat) for controllingthe temperature of the heating medium. The clad and the monomer can alsobe heated by far infrared rays, microwave or the like.

[0049] The container of the illustrated heating bath 13 (water bath forpolymerization) is equipped with two opening portions so that the clad 1can penetrate through the container. As shown in the drawing, oneopening portion is located at a clad take-in feed end (feeding apparatusside), whereas, the other opening portion is located at a clad take-outend (winding apparatus side). In place of the container as shown in thedrawing, a container having an opening portion only at one end in thelengthwise direction of the container. In this case, the container isdisposed along the generally vertical direction so that one opening ofthe container faces upward in the vertical direction. The clad is fed sothat a sealed end of the clad enters from the opening portion and, afterreversing the direction in the vicinity of the bottom (lower portion inthe vertical direction) of the container, the sealed end comes out fromthe opening portion again. In such way, the clad filled with the monomeris dipped in a heating medium and, after the completion of formation ofa core and shrinkage of the clad, a light fiber is taken out from theopening portion. Consequently, a long size light fiber can also becontinuously produced.

[0050] As shown in the drawing, a monomer filling tank 14 containing areactive filler material (polymerizable monomer in this embodiment) isprepared, and the clad 1 is filled with a polymerizable monomer ascontents under the applied pressure of a nitrogen gas (N₂).

[0051] In case the light fiber is continuously produced while feedingand winding the clad extending continuously in the lengthwise direction,it is advantageous to continuously fill with the monomer under pressure,like the embodiment shown in the drawing. In case the clad is filledwith the monomer under pressure in such way, one end of the clad in thelengthwise direction is usually sealed and the clad is filled with themonomer from the other end of the clad. The clad can be sealed byfitting a tap or valve made of glass, rigid plastic or metal (e.g.stainless steel) into the opening portion at one end of the clad.

[0052] On the other hand, the clad is filled with the monomer form theopening portion at the other end of the clad. As shown in the drawing,the clad is continuously filled with the monomer by bringing the openingportion at the other end of the clad 1 into contact with the monomer(usually in a liquid form) in the monomer tank 14 and maintaining theinside of the monomer tank 14 at the positive pressure. That is, suchcontinuous filling under pressure can be conducted by bringing an inertgas such as nitrogen into contact with the liquid level of the monomerwhile dipping the other end of the clad in the liquid monomer.

[0053] Subsequently, the clad 1 is fed from the feeding apparatus 12,thereby feeding the monomer, with which the clad 1 is filled, to theheating region in the water bath 13 for polymerization. At this time, incase the temperature of the water bath 13 for polymerization is uniformand the water bath for polymerization is a temperature controlled bathwhich is controlled to the temperature suited for polymerization of themonomer and shrinkage of the clad, the whole water bath serves as theheating region.

[0054] The clad 1 filled with the monomer is passed through the waterbath 13 for polymerization, for example, by connecting one end of aleading member such as wire, rope, tube or the like in the lengthwisedirection to the sealed end of the clad, setting the other end of theleading member to the core tube of the winding apparatus 12, and drivingthe winding apparatus 12. Alternatively, the portion having apredetermined length, which extends from the sealed end of the clad 1 toone end in the opposite side, is allowed to serve as a waist portion andthe waist portion can be utilized in place of the leading member.

[0055] In the embodiment shown in the drawing, the reaction (thermalpolymerization) of the monomer is initiated in the clad fed into theheating region and the polymerization is completed in the heatingregion. On the other hand, the clad fed into the heating region isexpanded by the pressure applied through the monomer. Since such a clad,which is expandable under pressure, is used, the clad can be uniformlyshrunk with volume reduction, which accompanies the monomer reaction, byheating in the heating region before the completion of the reaction ofthe monomer.

[0056] In the illustrated embodiment, the feed rate and the heatingtemperature of the clad are decided so that the polymerization (thermalpolymerization) of the monomer is completed in the water bath forpolymerization (heating region). Consequently, the reacted (polymerized)portion of the clad is located in the water bath even after thecompletion of the reaction of the monomer, thereby making it possible toheat the reacted portion. The clad can be shrunk uniformly in the stateof being closely contacted with the core by such a heating operation.

[0057] As described above, according to the production method of theillustrated embodiment, expansion of the clad, initiation and completionof the polymerization of the core (formation of solid core), andshrinkage of the clad can be conducted in a continuous and smoothmanner. In the water bath 13 for polymerization, thermal expansion ofthe clad proceeds in the zone A and thermal polymerization of the coreproceeds in the zone B and, furthermore, thermal shrinkage of the cladproceeds in the zone C.

[0058] The mechanism and effect of the shrinkage of the clad, whichaccompanies the polymerization of the monomer, can be described asfollows.

[0059] 1. The clad filled with the polymerizable monomer under pressureis expanded under pressure in the water bath for polymerization. Theexpansion operation of the clad can be conducted in an easy and uniformmanner by heating.

[0060] 2. The polymerization of the expanded polymerizable monomer inthe clad is initiated by heating. Volume reduction of the monomer duringthe polymerization, which occurred at the initial stage of thepolymerization, can be compensated by the monomer to be moved from theunpolymerized portion by the pressure applied to the unpolymerizedportion of the monomer.

[0061] 3. When the polymerization of the monomer proceeds and thefluidity (mobility) is lost, the monomer whose mobility was lost forms awall against the pressure. The portion extending further from the wall(discharge side of water bath) becomes a region where an influence ofthe pressure from the unpolymerized portion is not exerted.

[0062] 4. Until the polymerization is completed, volume reduction of thecore, which accompanies the polymerization reaction, continues. At thisstage, since filling with the monomer from the unpolymerized portionside is inhibited by the wall, volume reduction occurs in the radialdirection of the core. At this stage, the clad expanded under pressureis shrunk with volume reduction of the core and is closely contactedwith the core. At this time, heating of the clad in the water bath makesthe shrinking operation of the clad uniform and smooth. Accordingly, airgap can be effectively prevented.

[0063] The fiber 10 thus obtained after the completion of formation ofthe solid core and shrinkage of the clad is fed to the winding apparatus12, which is disposed outside the heating region 13 (water bath forpolymerization), where the fiber is wound. The light fiber 10 fedbetween the water bath 13 for polymerization and the winding apparatus12 is usually cooled slowly by an air in the production room.

[0064] When the heating temperature in the water bath for polymerizationis comparatively high (e.g. 75° C. or higher), it is preferred to windby the winding apparatus after passing through the slow-cooling regionwhose temperature is set at a predetermined temperature. When theheating temperature in the water bath is comparatively high, it becomesimpossible to neglect thermal expansion of the solid core after thecompletion of the reaction, thereby making it possible to shrink theclad with shrinkage, which accompanies cooling of the core. As theslow-cooling region of the clad, the region of room temperature (usually25° C.) can be used by providing a long distance between the water bathand the winding apparatus, or an oven disposed between the water bathand the winding apparatus can be employed. The temperature of the ovenis usually set within a range from the heating temperature and roomtemperature.

[0065] The clad in the water bath for polymerization is preferablyretained in a generally linear form. That is, a line connecting the cladfeed end and the discharge end of the water bath container is generallya straight line. Consequently, it becomes easier to uniformly shrink theclad with volume reduction of the core made of a polymer of a monomer,which accompanies the reaction of the monomer. The light fiber producedby such a method can be substantially free from air gap between the cladand the core.

[0066] According to the method of the present invention, there can beproduced efficiently a long size light fiber, which is substantiallyfree from cellular defect, peel or the like, without using a specialoperation or step for volume reduction compensation of the reactedmonomer (polymerized monomer).

[0067] [Conditions for Production of Light Fiber]

[0068] As described above, according to the production method of thepresent invention, by controlling main production conditions, i.e.thickness of the clad, internal radius of the clad, physical propertiesof the clad, such as elastic modulus of clad material, pressure in theclad (pressure for pressurizing the filler material such as monomer) andtemperature of the heating bath (water bath for polymerization) in thestep of polymerizing the monomer in the clad, which is expandable underpressure, the clad is deformed (plastic deformation or elasticdeformation) on the polymerization reaction and the polymerizationreaction of the monomer can be conducted in the expanded clad. Bycontrolling the above-described production conditions, the clad can beshrunk with volume reduction of the core, which accompanies the reactionof the filler material, thereby making it possible to enhance theadhesion between the core and the clad.

[0069] The production conditions are preferably decided as follows.

[0070] (a) Expansion of the clad is controlled by appropriately decidingthe temperature in the vicinity of the fed port of the clad, physicalproperties of the clad and internal radius of the clad. Expansion of theclad is decided within a range from volume reduction, which accompaniesthe polymerization reaction of the monomer, to burst limit of the clad.

[0071] (b) The pressure in the clad is set so that volume reductioncaused by the initial polymerization of the core can be compensated bythe unpolymerized monomer.

[0072] (c) The temperature in the vicinity of the clad discharge port ofthe heating bath is decided so that the expanded clad can be shrunk withvolume reduction of the core occurred in the radial direction in uniformand smooth manner. Physical properties of such clad are also set withina range where such clad shrinkage can be conducted.

[0073] One embodiment of specific production conditions will bedescribed below.

[0074] The elastic modulus of the clad material is usually from 10 to700 MPa, preferably from 20 to 600 MPa, and particularly preferably from30 to 500 MPa. When the elastic modulus is too small, there is a fearthat the clad bursts in the production process if the clad internalpressure is enhanced, thereby making it difficult to enhance thepressure in the clad. When the pressure in the clad is not sufficientlyenhanced, it becomes difficult to make expansion of the clad larger thanvolume reduction of the core, which accompanies the polymerizationreaction of the monomer, and there is a fear that the clad can not beshrunk with volume reduction of the core, which accompanies thepolymerization reaction. To the contrary, when the elastic modulus istoo large, expansion of the clad under pressure can not be increasedsufficiently and there is a fear that the clad can not be shrunk withvolume reduction of the core, which accompanies the polymerizationreaction of the monomer. In the present specification, the elasticmodulus of the clad material is defined as a value at the heatingtemperature on an expanding operation under pressure.

[0075] The thickness of the clad is usually from 0.01 to 2 mm,preferably from 0.05 to 1.5 mm, and particularly preferably from 0.1 to1 mm. When the thickness is too small, there is a fear that the cladbursts if the pressure in the clad is sufficiently enhanced, therebymaking it difficult to enhance the pressure in the clad. To thecontrary, when the thickness is too large, there is a fear that itbecomes difficult to expand the clad under pressure, thereby making itimpossible to shrink the clad with volume reduction, which accompaniesthe polymerization reaction of the monomer.

[0076] The internal radius of the clad is usually from 1 to 15 mm,preferably from 1.5 to 13, and particularly preferably from 2 to 12 mm.When the internal radius is too small, there is a fear that it becomesdifficult to expand the clad under pressure, thereby making itimpossible to shrink the clad with volume reduction, which accompaniesthe polymerization reaction of the monomer. To the contrary, when theinternal radius is too large, there is a fear that it becomes difficultto enhance the pressure in the clad, thereby making it impossible toeffectively prevent generation of air gap in the initial stage of thepolymerization of the monomer.

[0077] The material of the clad is not specifically limited as far as itis a material having the elastic modulus as described above. Examplesthereof are polymers such as tetrafluoroethylene-hexafluoropropylenecopolymer, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoridecopolymer, trifluoroethylene-vinylidene fluoride copolymer,polymethylpentene, ethylene-vinyl acetate copolymer, vinyl acetate-vinylchloride copolymer and the like.

[0078] The upper limit of the pressure in the clad varies depending onphysical properties of the clad, but is usually 5 MPa, preferably 3 MPa,and particularly preferably 2 MPa. When the internal pressure in theclad is too high, there is a fear that the clad bursts during theproduction process.

[0079] The lower limit P_(L) of the pressure in the clad is usuallydecided by the following equation:

P _(L) =E×t/r  (1)

[0080] where E is an elastic modulus of a clad material and t is aninternal radius of a clad. When the pressure in the clad is smaller thanthe lower limit P_(L) decided by the equation (1), there is a fear thatthe clad can not be shrunk with volume reduction, which accompanies thepolymerization reaction of the monomer, because of too small expansionof the clad under pressure. On the other hand, when the pressure in theclad is too small, there is a fear that volume reduction caused by theinitial polymerization of the polymer can not be compensated by theunpolymerized monomer, thereby making it impossible to effectivelyprevent generation of air gap. Accordingly, the pressure in the cladsatisfies the equation (1) and is usually set to 0.05 MPa or more,preferably 0.07 MPa or more, particularly preferably 0.1 MPa or more.

[0081] The temperature of the heating bath (water bath forpolymerization) varies depending on the reactivity of the monomer,physical properties of the clad, and feed rate (i.e. residence time inthe heating bath) of the clad filled with the monomer, but is usuallyfrom 35 to 90° C., and preferably from 40 to 85° C. When the temperatureis too low, there is a fear that expansion and shrinkage of the clad cannot be conducted uniformly and smoothly, thereby making it possible toshrink the clad with volume reduction of the core. To the contrary, whenthe temperature is too high, there is a fear that it becomes impossibleto neglect thermal expansion of the core after the completion of thereaction, thereby making it difficult to shrink the clad with volumereduction after removing from the heating bath. As described above, ashrinking operation of the clad can also be conducted effectively byparticularly providing the slow-cooling region of the clad. However, itis preferred that such a slow-cooling region is not providedparticularly so as to simplify the production process and operation,thereby to enhance the production efficiency. From such a point of view,the temperature of the heating bath is preferably within a range from 45to 65° C.

[0082] The time of the clad filled with the monomer to be resided in theheating bath (residence time) is not specifically limited, but isusually from 10 minutes to 5 hours, and preferably from 15 minutes to 3hours. The length of the clad used for production is usually from 10 to3000 m, and preferably from 20 to 2000 m.

[0083] [Light Fiber]

[0084] The light fiber according to the present invention comprises aclad formed as described above, and a core coated with the clad.

[0085] The core usually extends in the lengthwise direction of the core.The peripheral surface of the core is coated with the clad with beingclosely contacted with the core, both ends of the core being exposed.Light from a light source can be introduced into the core from exposedone end or both ends of the core without causing loss. The core haslight transmittance in such a level as to make it possible to transmitincident light incident upon the core from one end to the other end.

[0086] The core is a solid core made of a pliable plastic. As thepliable plastic, for example, polymers having light transmittance andpliability, such as acrylic polymer, ethylene-vinyl acetate copolymer,vinyl acetate-vinyl chloride copolymer and the like, can be preferablyused alone. Alternatively, two or more kinds of them can be used incombination. The refractive index of the plastic constituting the coreis usually from 1.4 to 1.7, while the total light transmittance isusually 80% or more. The polymer can be crosslinked to secure sufficientheat resistance of the core itself.

[0087] As the filler material as a raw material of the core, forexample, an acrylic monomer can be used. Specific examples of theacrylic monomer include n-butyl methacrylate, triethylene glycoldimethacrylate, methyl methacrylate, methyl acrylate, 2-hydroxyethylmethacrylate, n-propyl methacrylate, phenyl methacrylate, 2-ethylhexylmethacrylate, ethyl acrylate, tridecyl methacrylate, dodecylmethacrylate and the like.

[0088] As a crosslinking agent for crosslinking the material of thecore, for example, polyfunctional monomers such as diallyl phthalate,triethylene glycol di(meth)acrylate, diethylene glycol bisallylcarbonate and the like can be used.

[0089] The length of the core in the light fiber is usually the same asthat of the clad, and is usually from 50 to 100 m. Usually, the crosssection of the core in the radial direction is generally in a circularor ellipsoidal form, but may be in any other shape as far as the effectof the present invention is not impaired.

[0090] The light fiber according to the present invention can enhancethe light emission efficiency while sufficiently utilizing conventionalcharacteristics (flexibility and durability) of the light fiber.Accordingly, the light fiber can be used, for example, as constituentparts of light emitting devices which can be suitably used inilluminator in the place separated from a light source, illumination,advertising display, variable display and road traffic sign. That is,the light fiber according to the present invention can effectively usedin an end light emitting mode capable of emitting light, introduced intothe core from one end, from the other end, and a side light emittingmode capable of emitting (light leakage) light from the side (peripheralsurface) of the core.

[0091] As the light source, for example, high luminance lumps such asxenone lamp, halogen lamp, flush lamp and the like can be usedadvantageously. Waste of electric power of the lamp is usually from 10to 500 W. Sunlight may be introduced into the core from one end afterfocusing.

EXAMPLES

[0092] The present invention will be described with reference to thefollowing examples and comparative examples.

Examples 1 to 5

[0093] Under the production conditions described in the following Table1, optical fibers of the respective examples were produced in accordancewith the following production procedure. A production apparatus used ineach example was the same as that described previously with reference toFIG. 1 and was equipped with a water bath for polymerization disposedhorizontally (length in horizontal direction: 4.2 m). In Example 5wherein the temperature of the water bath for polymerization was raisedto 80° C., the resulting product was wound around a winding apparatusafter a slow-cooling operation of a clad was carried out.

[0094] [Production Procedure]

[0095] As a clad material, a tetrafluoroethylene-hexafluoropropylenecopolymer manufactured from Mitsui Du Pont Co. under the trade name ofFEP100J was used. This clad material was molded by using an extruderhaving a diameter (φ) of 50 mm and L/D of 26 to obtain a long size cladtube.

[0096] The resulting clad was cut into pieces having a length of 30 m,and the cut clad was filled with a core forming material under pressure.The core forming material used herein is that prepared by addingbis(4-t-butylcyclohexyl)peroxydicarbonate as a polymerization initiatorto a polymerizable monomer (mixed solution of n-butyl methacrylate andtriethyleneglycol dimethacrylate in a weight ratio of 100:1). Themonomer in the clad was polymerized under the conditions as described inthe following Table 1. In that case, the monomer was continuouslypolymerized in the clad, and the clad and the core were closelycontacted with each other by shrinkage of the clad to produce a desiredlight fiber.

[0097] [Produced Light Fiber]

[0098] In each of Examples 1 to 5, the clad can be shrunk on volumeshrinkage, which accompanies the polymerization reaction of the monomer,thus obtaining a light fiber wherein any air gap is not observed betweenthe core and the clad (that is, the number of air gap defined describedabove is zero).

Comparative Example 1

[0099] In Comparative Example 1, a light fiber was produced in the samemanner as in Example 1, except that the same clad as in Example 1 wasused and the pressure in the clad was lowered. The production conditionsare as described in the following Table 1.

[0100] Since expansion of the clad was not sufficient as compared withthe volume reduction of the monomer, the clad could not be shrunk withvolume reduction of the core, which accompanies the reaction of thefiller material. As a result, the number of air gap measured by themethod described above was about 10 per 10 m.

Comparative Example 2

[0101] In Comparative Example 2, a light fiber was produced in the samemanner as in Example 1, except that the same clad as in Example 3 wasused and the residence time in the water bath for polymerization wasreduced to half. The production conditions are as described in thefollowing Table 1.

[0102] In this example, the polymerization of the monomer was notcompleted in the water bath for polymerization and volume reduction ofthe core occurred with proceeding of the polymerization after removingfrom the water bath for polymerization. After removing from the waterbath for polymerization, the clad could be shrunk dimensionally,however, the clad could not be shrunk with volume reduction of the coreat room temperature (about 25° C.) outside the water bath forpolymerization. As a result, the number of air gap measured by themethod described above was about 10 per 10 m. By further heating afterremoving from the water bath for polymerization under the conditions ofComparative Example 2, it becomes possible to shrink the clad withvolume reduction of the core, thereby forming a light fiber which isfree from air gap. TABLE 1 Comp. Comp. Example Example Example ExampleExample Example Example 1 2 3 4 5 1 2 Elastic modulus of MPa 361.6 361.6361.6 361.6 154.62 361.6 361.6 FEP: E Temperature of ° C. 55.0 55.0 55.055.0 80.0 55.0 55.0 polymerization bath: T Residence time in h 1.0 1.01.0 1.0 1.0 1.0 0.5 polymerization bath: t Diameter: D mm 5.50 9.8012.80 18.80 12.80 5.50 12.80 Thickness of clad mm 0.25 0.35 0.35 0.450.50 0.25 0.35 material: t Nitrogen pressure: P MPa 0.55 0.45 0.35 0.300.35 0.35 0.35 Internal radius: r mm 2.75 4.90 6.40 9.40 6.40 2.75 6.40Thickness/internal — 0.09 0.07 0.05 0.05 0.08 0.09 0.05 radius: t/r

[0103] As described above, according to the present invention, since afiller material was reacted in a clad expanded under pressure and theclad was shrunk in conformity with volume reduction of the core, whichaccompanies the reaction of the filler material, the portion of theboundary between the polymerized portion and the unpolymerized portionin the clad is formed in a comparatively large width during the reactionof the core (monomer), thereby making it possible to substantiallyeliminate air gap at the interface between the core and the clad even incase where volume reduction of the polymerized core occurs in the rangewhere an influence of the pressure from the unpolymerized portion sideis not exerted. Accordingly, according to the present invention, a longsize light fiber can be continuously produced in a stable and easymanner.

1. A light fiber comprising (a) a tubular clad having a predeterminedlength and (b) a solid core formed by reacting a filler material withwhich the clad is filled, the clad and the core being closely contactedwith each other by shrinkage of the clad, characterized in that: theclad is expandable under pressure, shrinkage of the clad is initiatedbefore the completion of the reaction of the filler material and iscarried out in conformity with volume reduction of the core, whichaccompanies the reaction of the filler material, and the number of airgap between the clad and the core, which is measured per length of 10 m,is 3 or less.
 2. A method of producing a light fiber comprising (a) atubular clad having a predetermined length and (b) a solid core formedby reacting a filler material filled in the clad, the clad and the corebeing closely contacted with each other by shrinkage of the clad, saidmethod comprising the steps of: forming the clad which is expandableunder pressure, filling the clad with the filler material and reactingthe filler material in the clad while applying a pressure, initiating ashrinking operation of the clad before the completion of the reaction ofthe filler material, and shrinking the clad with volume reduction of thecore, which accompanies the reaction of the filler material, therebyinhibiting generation of air gap between the clad and the core.
 3. Themethod of producing a light fiber according to claim 2, wherein thelight fiber is continuously produced by the steps of: (A) feeding theclad from a feeding means, on which the clad is wound, using windingmeans, thereby transferring the filler material, with which the clad isfilled, to a heating region, (B) initiating, carrying out and completingthe reaction of the filler material in the heating region, (C)initiating a shrinkage operation of the clad in the heating region, and(D) winding the resulting light fiber after the shrinking operation ofthe clad has substantially been completed, using a winding meansdisposed outside the heating region.